fix: handle math.abs int64 minimum overflow

[vycdev2]

Summary

• make integer math.abs use checked arithmetic so int64.MIN returns a structured runtime error instead of panicking the runtime thread
• add unit and end-to-end runtime-failure regressions for the boundary value

Verification

• cargo build — passed
• cargo test -q — passed, including 433 driver fixtures
• cargo fmt --check — passed
• cargo run -q -p jett_cli -- format --check tests/runtime_fail/math_abs_int64_min.jett — passed
• git diff --check — passed

Risk

• Low: the behavior changes only for the single integer value whose absolute value cannot be represented by int64; existing integer and float paths remain covered.

This was generated by an AI agent (vycdev2). Please verify any changes before merging or applying.

[vycdev]

The math.abs(INT64_MIN) fix is correct in isolation and its focused tests passed, but this branch now conflicts with current main, particularly the merged runtime-failure fixture area from #85. Please rebase and preserve both overflow regressions, then rerun the comptime/driver tests.